1. Field of the Invention
The present invention relates to a method of detecting misfires in a gasoline internal combustion engine (hereinafter referred to merely as "an engine", when applicable).
2. Related Art
A conventional misfire detecting method of this type, as disclosed by Japanese Patent Application (OPI) No. Sho. 58-138271 (the term "OPI" as used herein means an "unexamined published application"), employs an angular velocity sensor for detecting the angular velocity of rotation of an engine, an arithmetic circuit for calculating the period of the angular velocity of rotation detected by the rotational angular velocity sensor, and a determining circuit for determining whether or not the difference between the average value of the periods thus calculated and the peak value thereof is less than a predetermined value, to detect the troubles of the engine which are due to the occurrence of misfires in the engine, the unsuitable ignition timing, abnormal vibrations, etc.
As noted above, in the conventional misfire detecting method, it is determined that a misfire has occurred when the difference between the average value of the periods of the angular velocity of rotation of the engine, and the peak value thereof exceeds the predetermined value (which is constant irrespective of the conditions of the engine). In general, in case of high rotation or light load, the angular velocity of rotation of the engine is less variable. Hence, in order to detect a misfire in the case where the velocity of rotation is high or the load is light, the threshold value for decision should be low. On the other hand, in the case where the speed of rotation is low or the load is heavy, the angular velocity of rotation varies greatly even if there is no misfire occurring in the engine. Hence, if, in this case, the threshold value is low, then it may be determined that an abnormal condition has occurred although no abnormal condition is, in fact, involved at all. If, on the other hand, the threshold value is set to a high value, then when the speed of rotation is high or the load is light, the occurrence of misfires cannot be detected.
In general, in the conventional misfire detecting method, the occurrence of misfires is decided when a condition for deciding the occurrence of misfires (hereinafter referred to as "a misfire deciding condition", when applicable) is satisfied even only once. Hence, when the angular velocity of rotation of the engine is unstable, the misfire deciding condition is satisfied even if no abnormal condition is involved, resulting in a determination that a misfire has occurred. In addition, in the conventional method, it is impossible to decide the occurrence of a misfire depending on the operating conditions of the engine.